There has been much work directed to providing frequency conversion of the output from presently available laser and laser diode sources to wavelengths not readily available from these sources. The most attractive alternative for frequency conversion, such as frequency doubling, sum frequency generation and difference frequency generation, is quasi-phase matching (QPM) of an input radiation beam or beams from laser or laser diode sources and their harmonic waves in second order optical crystals. Typical second order optical crystals for use in such applications include inorganic crystals such as, for example, LiNbO3, LiTaO3 and KTP. In the case of such crystals, the QPM conditions must be satisfied between the interacting waves in order to achieve efficient nonlinear optical interaction.
QPM allows interactions between lightwaves or radiation polarized such that the nonlinearity is maximized and allows interactions to occur in the crystal for which birefringent phase matching is not possible. Compared to birefringent phase matching, QPM allows both access to new wavelengths and higher conversion efficiencies. Since the refractive index of the crystal is dependent upon wavelength of the light to be converted, it is necessary to provide a periodic inverted domain structure (i.e., periodic poling) within the crystal so as to have domains in the crystal of nonlinear optical coefficient of periodic inverted sign, e.g., two or more regions or domains of different states of ferroelectric polarization transverse to the path of light to be converted. First order QPM requires sign reversals of the effective nonlinear coefficient with a period equal to two coherence lengths. The light waves produced by the nonlinear polarization periodic pattern in the crystal are in phase at the given wavelength so that the waves intensify each other.
To date, one frequency conversion that is highly desirable is that which generates visible light in the “blue” radiation spectrum, such as wavelengths in the range of about 390 nm to 492 nm, which has many applications such is in color display devices, color projectors and color printers.
In practice, the ability to create finely spaced domains with sufficiently accurate periodicity and well defined domain walls in the crystal is a challenging, if not difficult, task to accomplish, particularly on a continuous yield basis. So far, there are presently several ways to form the periodic domain pattern of desired spontaneous polarization in the nonlinear crystal, i.e., processing regions or domains having a ferroelectric polarization direction that is dominant over all other possible directions. These several ways may be classified, in part, as (1) inverted domain patterns of differing composition, i.e., by surface impurity diffusion or by ion exchange, (2) inverted domain patterns of same composition, i.e., electric field treatment with or without heat, (3) inverted domains through periodic modulation during crystal growth, i.e., current bias or temperature fluctuation treatment during crystal growth (e.g., by a modified Czochralski process) and (4) electron beam treatment.
KTP is poled most typically by an ion exchange process within the first classification. A chromium mask is evaporated onto the surface of the crystal. Looking through the mask is somewhat like looking through a black, plastic comb, with the teeth of the comb representing the presence of chromium. The masked crystal is placed in a melt of BaNO3 or RbNO3. Exchange of Ba or Rb for the K in the KTP occurs only where the chromium is absent; the chromium blocks the exchange where it is present. The mask is then removed. The resultant, periodic stripes where ion exchange has occurred have a different index of refraction than the pure KTP stripes.
The second type of classification is generally achieved by the application of a high voltage, electric field through the employment of a pattern of electrodes formed on one major surface of the crystal with a planar electrode formed on the opposite major surface of the crystal forming the opposing field electrode. The applied field is either pulsed or a continuous wave for a short period of time and is generally accompanied with an applied temperature such as above 100° C. The permanent inversion of the domains is accomplished by means of minute changes in ions in the unit lattice of the crystal due to the application of the electric field. By “permanent”, what is meant is that the inverted domain pattern will remain as long as the crystal is not subsequently reheated to high temperature near the Curie temperature of the crystal or subjected to any further high voltage fields.
In about 1963, R. C. Miller recognized that inverted domains could be formed in ferroelectric crystals by cycling an applied electric field to switch the spontaneous polarization of the crystal. U.S. Pat. No. 5,193,023 teaches periodic poling, using a pattern of electrodes on one side of a crystal and a planar electrode on the opposite side of the crystal across which an electric field is applied. In the examples of U.S. Pat. No. 5,193,023 where an electric field is employed, poling is accomplished in an atmosphere containing oxygen with an applied temperature in the range of 150° C. to 1200° C. and an applied voltage field of several hundreds of volts per centimeter or less. The field inversion in U.S. Pat. No. 5,193,023 is accomplished at relatively lower applied voltages, such as at several hundreds of volts per centimeter (or several kilovolts per centimeter when using pulse voltages) or less, since the crystal is heated to a sufficiently high temperature during the applied E-field process. However, it has been found that higher voltages can be successfully employed at room temperature.
Examples of the third type of classification are, respectively, the articles of A Feisst et al., “Current Induced Periodic Ferroelectric Domain Structures in LiNbO3 Applied for Efficient Nonlinear Optical Frequency Mixing”, Applied Physics Letters, Vol. 47(11), pp. 1125-1127, Dec. 1, 1985 and Duan Feng et al., “Enhancement of Second Harmonic generation in LiNbO3 Crystals With Periodic Laminar Ferroelectric Domains”, Applied Physics Letters, Vol. 37(1), pp. 607-609, Oct. 1, 1980. Both of these articles describe crystals grown by flux growth methods at temperatures above the Curie temperature of the crystal.
An example for the fourth type of classification is the article of H. Ito et al., “Fabrication of Periodic Domain Grating in LiNbO3 by Electron Beam Writing for Application of Nonlinear Optical processes”, Electronic Letters, Vol. 27(14), pp. 1221-1222, Jul. 4, 1991.
Of all of the foregoing classifications, heretofore the second type of classification has been found the most successful from the standpoint of providing periodic domains that have accurate periodicity and substantially vertically formed domain walls creating the nonlinear periodic waveguide in the crystal. The use of the applied electric field permits the formation of domains that have accurate periodicity and the domains are formed through the crystal forming domain walls that have some parallelism with the z axis of the crystal. However, in the case of the second type as well as all other types classified, the processing only provides for shallow domain structures that do not effectively extend through the crystal bulk and do not form vertical wall boundaries for the formed inverted domains substantially parallel with the z axis of the crystal. What is needed is a process that provides for vertically formed domain walls that extend in the z axis direction through the crystal bulk without walkoff, i.e., capable of providing bulk frequency conversion, forming highly uniform periodicity, laterally extending domain patterns which achieve first order intervals over long crystal interaction lengths. Heretodate, such domain patterns have only extended a maximum of about 3 mm into the crystal depth.
Although not known as a means for producing periodically poled crystals, hydrothermal techniques are an excellent and well known route to high quality single crystals for a variety of electro-optic applications. For example, all electronic grade quartz is grown commercially by the hydrothermal method. Further, KTP is grown by both flux and hydrothermal methods, and it is widely acknowledged by those familiar with the art that the hydrothermally grown product is generally of superior quality. The hydrothermal method involves the use of superheated water (liquid water heated above its boiling point) under pressure to cause transport of soluble species from a nutrient rich zone to a supersaturated growth zone. Generally, a seed crystal is placed in the growth zone to control the growth and supersaturation is achieved by the use of differential temperature gradients. The superheated fluid is generally contained under pressure, typically 5-30 kpsi, in a metal autoclave. Depending on the chemical demands of the system the autoclave can be lined with a noble metal using either a fixed or floating liner. These general techniques are well known to those of ordinary skill in the art and have been used for the growth of a variety of electro-optic crystals.